D-cycloserine facilitates extinction the first time but not the second time: an examination of the role of NMDA across the course of repeated extinction sessions

Pharmacological discrimination of extinction and reconsolidation of contextual fear memory by a potentiator of AMPA receptors

Conditioned fear memory, once formed through fear conditioning, is modulated by reexposure of individuals to a conditioned stimulus. The reexposure reactivates the fear memory, which induces reconsolidation of the memory first, and then extinction of the fear response. Both attenuating the former and facilitating the latter are effective in reducing the fear response, and these findings are potentially translatable to the enhancement of exposure therapy for complex anxiety disorders. Currently, there is no drug that is established to modulate either reconsolidation or extinction selectively, which are thought to be independent processes. Here, we report that an extinction-facilitating AMPA potentiator, 4-[2-(phenylsulfonylamino)ethylthio]-2,6-difluoro-phenoxyacetamide (PEPA), does not act on the reconsolidation of fear memory formed by contextual fear conditioning in mice. The freezing rates observed in contextually conditioned mice following short reexposure (3 min) to the context were not influenced by intraperitoneal or intra-amygdala administration of PEPA. The same short reexposure to the context enhanced freezing responses in mice that were similarly administered D-cycloserine (DCS), a drug that facilitates both extinction and reconsolidation, and this enhancement of freezing responses in mice intraperitoneally administered DCS was abolished by propranolol, a drug that suppresses reconsolidation. At the same doses used in the short reexposure experiments, PEPA and DCS facilitated extinction of the fear response induced by long reexposure to the context and suppressed reinstatement of the conditioned fear memory. PEPA and DCS did not affect reextinction. These results suggest that PEPA acts on extinction of contextual fear memory without having detectable influences on its reconsolidation.

The role of context in the re-extinction of learned fear

Extinction of learned fear is both amygdala- and NMDA receptor (NMDAr)-dependent. Recent studies, however, have shown that extinction the second time (re-extinction) does not involve the amygdala and is NMDAr-independent. The present study compared the effects of context change on extinction and re-extinction in adult Sprague-Dawley rats. Experiment 1 showed that both extinction and re-extinction are context-specific with a renewal effect occurring in both cases. Experiment 2 then examined whether the transition from an NMDAr-dependent to an NMDAr-independent process was context-specific. As expected, the results showed that MK-801 (0.1 mg/kg) impaired initial extinction but did not impair re-extinction (i.e., re-extinction was found to be NMDAr-independent). A novel finding was that if re-extinction occurred in a context different from initial extinction, then MK-801 impaired re-extinction. In other words, re-extinction is NMDAr-dependent (i.e., like initial extinction) when it occurs in a different context to initial extinction. Therefore, the switch from NMDAr-dependent to NMDAr-independent extinction is both stimulus [Langton, J.M., Richardson, R. (2008). D-cycloserine facilitates extinction the first time but not the second time: An examination of the role of NMDA across the course of repeated extinction sessions. Neuropsychopharmacology, 33, 3096-3102.] and context-specific (the present study). The precise conditions that govern whether extinction requires NMDAr activation are of considerable theoretical interest and remain to be fully characterized.

What does it take to demonstrate memory erasure? Theoretical comment on Norrholm et al. (2008)

An issue of increasing interest in Pavlovian conditioning is to identify ways to facilitate the development and persistence of extinction. Both behavioral and molecular lines of evidence demonstrate that learning during extinction can be enhanced. Similar evidence has been offered to support the idea that extinction causes the original association to be unlearned, or erased. Differentiating between extinction and erasure accounts is extremely difficult and requires many assumptions about the fundamental nature of how memory storage maps into memory expression. In this issue of Behavioral Neuroscience, Norrholm et al. (see record 2008-13280-002) describe a study of extinction with humans that has the potential to serve as a translational bridge between rodent work and clinical applications. They find less recovery of a conditioned fear response when extinction occurs 10 min compared with 72 hr after conditioning; however, the recovery of subjects' expectancies of the fearful stimulus is independent of when extinction occurred. These findings and others discussed in this article demonstrate some of the challenges in making inferences about memory erasure during extinction.